The technology of surface finishing has yielded many effective coatings which are well known to those skilled in the art. The use of zinc-rich coatings has found wide acceptance for mill application on metal (e.g., steel) coil. These coatings usually perform adequately until the coated surface is damaged, which typically causes a portion of the protective coating to be removed from the protected surface, making it vulnerable to corrosion. Restoration of corrosion protection to the damaged site must then be undertaken. The subsequent application of a zinc-rich coating is a recognized way to renew the corrosion protection of a damaged metal surface.
Zinc-rich coatings are applied to steel surfaces to provide galvanic and sacrificial protection. These coatings are desired because they are very effective, even in chemical environments. The zinc will be chemically used up or dissipated before the steel is attacked. Zinc-rich coatings also tend to self-heal, frequently preventing the spread of subsequent corrosion or rust.
Organic zinc-rich coatings have been used as weld-through coatings for many years. These coatings typically contain a minimum of 80 weight percent zinc pigment in a film-forming binder. Such a zinc loading is necessary to ensure the electrical conductivity of the coating. However, when the zinc-rich coating is subsequently subjected to high temperatures, e.g., during welding, the zinc-rich coating is usually vaporized with an attendent loss of the protection afforded by the zinc.
The present preferred method of welding in automotive body shops is gas metal arc welding because it offers more versatility than the other welding methods. Gas metal arc welding, which is often called MIG (metal inert gas) welding, is an arc welding process in which the heat for welding is generated by an arc between a consumable electrode and the work metal. The electrode, a bare solid wire that is continuously fed to the weld area, becomes the filler metal as it is consumed. The electrode, weld pool, arc, and adjacent areas of the base metal are protected from atmospheric contamination by a gaseous shield provided by a stream of inert gas, or mixture of gases, fed through the welding gun. The gas shield must provide full protection because even a small amount of entrained air can contaminate the weld deposit.
Difficulties arise in the welding of steels that are coated with low melting point metals such as zinc, as in a zinc-rich coating. These coated surfaces are difficult or impossible to weld satisfactorily because the welding heat causes fuming of the coating or alloying with the base metal, or both. This results in welds with poor mechanical strength or environmental resistant properties. When coated metals are to be welded, the coating is then usually first thoroughly removed from the joint areas. Post weld repair of the coating over the weld area is then required to afford corrosion protection. Application of an anti-corrosion coating to the overlap area of the welded lap joint to provide adequate corrosion protection is very difficult to achieve.
U.S. Pat. No. 4,417,007 assigned to the Elkem Metals Co. of Pittsburgh, Pa., discloses the use of manganomanganic oxide fume pigment in a zinc-rich paint formulation which substantially improves the corrosion resistant properties of these zinc-rich paints. This coating merely acts as a protective coating--not as a weld-through coating.